There have already been proposed by the applicant of the present application techniques for controlling the air-fuel ratio of an air-fuel mixture to be combusted by an internal combustion engine for converging the output of an exhaust gas sensor, e.g., an O2 sensor (oxygen concentration sensor), disposed downstream of a catalytic converter, to a predetermined target value (constant value) in order to achieve the appropriate purifying capability of the catalytic converter, such as a three-way catalyst or the like, disposed in the exhaust gas passage of the internal combustion engine (e.g., see Japanese laid-open patent publication No. 11-324767, and Japanese laid-open patent publication No. 2000-179385).
According to these techniques, an exhaust system ranging from a position upstream of the catalytic converter to the O2 sensor disposed downstream of the catalytic converter is an object to be controlled which has an input quantity represented by the air-fuel ratio of the exhaust gas that enters the catalytic converter and an output quantity represented by the output of the O2 sensor. A manipulated variable which determines the input quantity of the exhaust system, e.g., a target value for the input quantity of the exhaust system, is sequentially generated by a feedback control process, or specifically an adaptive sliding mode control process, for converging the output of the O2 sensor to the target value, and the air-fuel ratio of the air-fuel mixture to be combusted by the internal combustion engine is controlled depending on the manipulated variable.
Generally, the behavior and characteristics of the exhaust system vary depending various factors including the operating state of the internal combustion engine. The exhaust system including the catalytic converter has a relatively long dead time.
According to the above techniques, the behavior of the exhaust system is modeled by regarding the exhaust system as a system for generating the output of the O2 sensor from the air-fuel ratio of the exhaust gas that enters the catalytic converter via a dead time element and a response delay element, and a parameter of the model of the exhaust system (a coefficient parameter relative to the dead time element and the response delay element) is sequentially identified using sampled data of the output of the O2 sensor and sampled data of the output of an air-fuel ratio sensor that is disposed upstream of the catalytic converter for detecting the air-fuel ratio of the exhaust gas that enters the catalytic converter. The manipulated variable is sequentially generated using the identified value of the parameter of the model according to a feedback control process that is constructed based on the model.
According to the above techniques, the process of identifying the parameter of the model of the exhaust system and the feedback control process using the identified value of the parameter are carried out to compensate for the effect of behavioral changes of the exhaust system and smoothly perform the control process for converging the output of the O2 sensor to the target value, or stated otherwise, an air-fuel ratio control process for achieving an appropriate purifying capability of the catalytic converter.
According to the above techniques, basically, the dead time of the exhaust system is regarded as of a constant value, and a preset fixed dead time is used as the value of the dead time of the dead time element in the model of the exhaust system.
The inventors of the present application have found that the actual dead time of the exhaust system varies depending on the state, such as the rotational speed, of the internal combustion engine, and the range in which the dead time of the exhaust system is variable may become relatively large depending on the operating state of the internal combustion engine. Consequently, depending on the operating state of the internal combustion engine, an error between the model of the exhaust system and the behavior of the actual exhaust system may become large. Because of this error, an error and a variation of identified value of the parameter of the model of the exhaust system become large.
According to the above techniques, since a highly stable control process such as an adaptive sliding mode control process is used as the feedback control process for generating the manipulated variable, it basically is possible to avoid a situation where the stability of the control process for converging the output of the O2 sensor to the target value would significantly be impaired.
In circumstances where the error and variation of the identified value of the parameter of the model of the exhaust system is relatively large, however, when the manipulated variable is generated using the identified value and the air-fuel ratio of the air-fuel mixture is manipulated depending on the manipulated variable, the output of the O2 sensor tends to vary with respect to the target value, and the quick response of the control process converging the output of the O2 sensor to the target value is liable to be lowered. This has presented an obstacle to efforts to further increase the purifying capability of the catalytic converter.
The present invention has been made in view of the above background. It is an object of the present invention to provide an apparatus for and a method of controlling the air-fuel ratio of an internal combustion engine to stably determine a highly reliable identified value of a parameter of a model of an exhaust system including a catalytic converter and hence to increase the purifying capability of the catalytic converter in a system for manipulating the air-fuel ratio to converge the output of an exhaust gas sensor such as an O2 sensor or the like disposed downstream of the catalytic converter to a predetermined target value to achieve an appropriate purifying capability of the catalytic converter. It is also an object of the present invention to provide a recording medium storing a program for controlling an air-fuel ratio appropriately with a computer.